Nick Haycock pulled no punches after the Development Squad's 5-2 Barclays Under-21 Premier League defeat at Southampton, bemoaning his side's severely under-par second-half showing.
Having twice taken a first-half lead, West Ham United went in all square at the break, to then fall away in dramatic fashion at the Saints' Staplewood Training Ground.
Sam Hoskins, Omar Rowe and Jake Sinclair each profited from careless defending to put Southampton well out of sight and end the Hammers' hopes of gate-crashing the Elite League's top three.
The result was made all the more frustrating by the Hammers' excellent start and Haycock was left wondering what might have been, had his side taken full advantage of a series of clear-cut early chances.
He told West Ham TV: "We'll look back at that first 20 minutes and wonder how we've lost the game at all, never mind by that scoreline. For those 20 minutes the boys played to instructions, got out the traps and hit them hard early. We were a threat doing things the right way and we could have been 3-0 up before they'd even started.
"Sam Allardyce has been saying it with the first team, when you create the chances you have to take them. At the top level you end up being punished if not, because you're playing against good sides, well organised teams with good individuals."
Haycock was forced into a double swap at the break, with both Dan Potts and Ben Marlow having to be withdrawn, and he conceded that his side struggled to get to grips with the new personnel and shape. That aside, he was in little doubt that the better side on the day had prevailed.
He continued: "I think [the substitutions] maybe unsettled us a little bit at the start of the second half. But firstly, the boys have got to be ready to come on and secondly the team has to adapt to what was just a slight change in shape.
"No excuses, they played better than us. Sometimes as a coach you have to hold your hands up and say the second 45 minutes was totally dominated by Southampton. They looked hungrier, they looked more intelligent and they looked like they wanted to win the game.
"It's the first time they have let me down and I told them that in no uncertain terms. The performance levels in the second half were unacceptable."
Chief of Haycock's complaints was his side's uncharacteristically charitable defending, such that the Hammers' boss admitted the visitors were fortunate to have only conceded five in the end.
"All the goals were avoidable. Sometimes the ball has got to go. We can play but you've also got to defend. Good performances start from the back with keeping a clean sheet. It has stood the first team in good stead this year and I've told the boys that if you're a defender at this club, your job is to defend. The goals were kamikaze.
"The boys just didn't perform second half and that's what happens. If your level drops in the Elite League, you're going to get beaten and you could get beaten heavily. I think they were lucky to come off only 5-2."
The Hammers travel to West Bromwich Albion on Tuesday for the season finale and though they can now finish no higher than fifth, Haycock is determined to end a positive campaign on a high.
"We can't qualify, but in my heart of hearts I expect Liverpool to pick up the points they need anyway. It would have been nice to take it to the last game.
"I don't want the season to witter away. I want a reaction as a group of players and we have to look to West Brom to make sure we finish the season strongly."